Sharing is one of the hallmarks of human behavior: give me a cookie and I’m more likely to give you one later. But our bonobo cousins have an odd variation on the practice. They share with strangers before friends. The finding is in the journal PLoS ONE. [Jingzhi Tan and Brian Hare, Bonobos Share with Strangers]

Researchers tested bonobo sharing in experiments involving fourteen of the apes. All were born in the wild. In the primary experiment, bonobos were placed in a cage with food, and they could choose to admit either a known member of their group, a stranger, or both. In 51 trials, most bonobos shared the feast, but they let the stranger in first.

Why choose an outsider over a friend? In another experiment, the scientists found bonobos only shared when doing so led to a social interaction. Giving up some food to strangers lets these apes expand their social network. This behavior may have evolved to promote social tolerance, in contrast with chimps' sometimes deadly aggression against strangers. Which means that even when food is offered, there's still no such thing as a free lunch.